This invention relates to a power source control device having a plurality of sources for supplying DC voltage or a plurality of signal sources, and more particularly to a power source control device which sequentially controls changes in the voltages supplied by the DC voltage or signal sources when a commercial power supply for powering the system is turned on or off.
Bubble memory units have recently been used in data memory devices for numerical control units and the like because they are cheap and, more importantly, because they are non-volatile and thus permit the stored data to be preserved even if commercial power is lost. Bubble memory units generally include a bubble memory element, a control unit for controlling such operations as the writing and reading of information with respect to the bubble memory element, and a stabilized DC power source for supplying the power to the bubble memory element and the control unit. The bubble memory element is generally of the type that is actuated by a magnetic field and includes, therefore, coil drive circuitry and associated control circuitry. The control unit includes a sense amplifier, read/write control circuitry and various other circuitry, and is adapted to write into the bubble memory element data which is sent from an external unit such as a numerical control device, or to read out the stored content of the bubble memory element and deliver this data to the external unit.
The stabilized DC power source device produces a memory enable signal M.sub.e, a first DC voltage E.sub.c which is supplied to the bubble memory element and to the control circuitry in the control unit, and a second DC voltage E.sub.d which is supplied to the coil drive circuitry of the bubble memory element. When the respective levels of the first and second DC voltages E.sub.c, E.sub.d are higher than specified levels, and when the level of the enable signal M.sub.e also is higher than a specified level (i.e., at logic level 1), the data obtained from the external unit can be written in the bubble memory element, or data stored in the element can be read out and sent to the external unit. Furthermore, the control unit is operable to deliver a read/write instruction to the bubble memory element when the level of the memory enable signal M.sub.e is higher than a specified level. However, neither the reading nor writing of data is possible if either of the first or second DC voltages E.sub.c, E.sub.d is at a level below the specified level, even if the level of the memory enable signal M.sub.e is higher than the specified level, that is, even if the signal M.sub.e is a logic 1.
On the other hand, after the control unit has delivered a read/write instruction, even though the level of the signal M.sub.e goes below the specified level, if the respective levels of the first and second DC voltages E.sub.c, E.sub.d are higher than the specified levels, the control unit can write into the bubble memory element data being temporarily stored in the control unit.
Data can be read-out from certain bubble memory elements by destructive reading, in which the stored data is destroyed as it is read out. This is followed by a refreshing operation in which the data which has been read out is written back into the bubble memory, thereby allowing the data to be preserved. The writing of data is carried out by replacing the content at a prescribed address with all zeros (an erase cycle), and then by writing the desired data into the location identified by the address (a write cycle). Accordingly, in the destructive reading of data from the bubble memory, a serious situation can occur in which the stored data is lost if the first and second DC voltages E.sub.c, E.sub.d drop below the prescribed levels after the data has been read out, but before it has been refreshed. Such a situation can arise if there is a power failure or if the commercial power supply is interrupted accidentally. The loss of data in this manner is a frequent occurrence since a considerable amount of time is required between the completion of the destructive reading step and the completion of the refreshing step. A similar danger arises in the writing of the data as well. In other words, the stored data can be lost if the first and second DC voltages E.sub.c, E.sub.d drop below the prescribed levels before the data is written into the bubble memory after the content at the prescribed address has been replaced by zeros. Again, this can be caused by a power failure or interruption of the commercial power supply, and the possibilities of such an event are high because of the considerable amount of time which elapses between the erase cycle and the completion of the write cycle.
Accordingly, if commercial power is lost because of carelessness or because of a power failure, it is required that the enable signal M.sub.e be lowered to a prescribed level a predetermined period of time after the loss of commercial power, but that the first and second DC voltages E.sub.c, E.sub.d be maintained above specified values for a certain period of time. This is necessary to prevent the destruction of the stored data in the event that the commercial power is lost. In other words, if power is lost, the signals M.sub.e, E.sub.c and E.sub.d must follow a certain sequence in dropping below specified levels, in order to preserve the stored data. The sequence begins with the enable signal M.sub.e. Next, the second DC voltage E.sub.d must fall after a predetermined period of time, which must be longer than that required for a refresh or write cycle. Finally, the first DC voltage E.sub.c must fall. Moreover, when the commercial power is restored, data sent from the external unit can be stored in the bubble memory element in a reliable manner if the signals M.sub.e, E.sub.c and E.sub.d rise above their respective specified levels in the order E.sub.c, E.sub.d, M.sub.e as they undergo the transition in the positive direction.
In a conventional apparatus the first DC voltage E.sub.c is maintained at a level above a prescribed value for an extended period of time by means of a battery when the commercial power is lost, and the enable signal M.sub.e and second DC voltage E.sub.d are caused to fall below their respective specified levels in succession during this period of time. Another conventional arrangement includes a sensor which senses a fluctuation in the voltage delivered by the commercial supply. As soon as an abnormality is detected in the commercial supply the memory enable signal is sent to a low level and the driving power sources are turned off. However, the first method is disadvantageous since the battery is large and fairly expensive. The second method is defective if employed where there are violent fluctuations in voltage, such as occur in a machine shop. This is because the sensor is actuated even by small drops in power source voltage and responds by turning off the power source device for the bubble memory unit. This results in the shut-down of equipment such as machine tools that rely upon the bubble memory unit, and a decline in the efficiency of machine tools controlled by a numerical control device.